1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, an LCD device having a polarizing plate for improving luminance and preventing backlight Mura phenomenon.
2. Discussion of the Related Art
With development of the present information society, the demand for various display devices has increased dramatically quite recently. Accordingly, much effort has been expended to research and develop various flat display devices, such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD) devices. Some of these flat display devices have already been used in displays of various devices.
Among the various flat display devices, the liquid crystal display (LCD) device has been most widely used due to its numerous advantages. LCD devices are thin, lightweight, and have a relatively low power consumption compared with the other types of displays, most notably Cathode Ray Tubes (CRT). This allows the LCD to substitute for the CRT in most devices. In addition to LCDs incorporated in mobile devices such as being used as a display for a notebook computer or personal data assistant (PDA), LCD devices have been developed for stationary electronic devices such as computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology with applications in different fields, research in enhancing the picture quality of the LCD device has been in some respects lacking as compared to other features and advantages of the LCD device. In order to use the LCD device in various fields as a general display, the key to developing the LCD device lies on whether the LCD device can implement a high quality picture, such as high resolution and high luminance with a large-sized screen while still maintaining lightness in weight, thinness, and low power consumption.
The LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel includes lower and upper glass substrates bonded to each other at a predetermined interval, and a liquid crystal layer injected between the lower and upper glass substrates. At this time, the liquid crystal layer is driven according to an electric field between the lower and upper substrates, thereby controlling light transmittance. As a result, the picture image is displayed on the LCD panel.
Hereinafter, a related art LCD device will be described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view illustrating the related art LCD device. As shown in FIG. 1, a lower substrate 10 of the related art LCD device includes a plurality of pixel regions (not shown) in a matrix type, and a thin film transistor (not shown) and a pixel electrode 11 formed in each pixel region. Also, an upper substrate 1 includes a color filter layer 3 for displaying various colors, and a common electrode 5. Then, a liquid crystal layer 13 is formed between the lower and upper substrates 10 and 1. Subsequently, first and second polarizing plates 14a and 14b are respectively formed on the upper substrate 1 and under the lower substrate 10 for linearly polarizing visible light, and a backlight unit 15 is formed under the second polarizing plate 14b. 
Although not shown, a plurality of gate lines are formed on the lower substrate (TFT array substrate) 10 at fixed intervals, and a plurality of data lines are formed perpendicular to the gate lines at fixed intervals, thereby defining the plurality of pixel regions. Then, the plurality of pixel electrodes 11 are respectively formed in the pixel regions as the matrix type, and the plurality of thin film transistors are switchable in response to signals of the respective gate lines for transmitting signals of the respective data lines to the respective pixel electrodes 11. After that, a first alignment layer 12 is formed to determine an alignment direction of liquid crystal. Also, the upper substrate (color filter substrate) 1 includes a black matrix layer 2 for excluding light from portions of the lower substrate except in the pixel regions, a Red/Green/Blue color filter layer 3 for displaying the various colors, the common electrode 5 on an entire surface of the upper substrate 1 for obtaining a picture image, and a second alignment layer 6 on the common electrode 5 for determining the alignment direction of the liquid crystal. An overcoat layer 4 protects the color filter layer 3 and flattens the upper substrate 1.
FIG. 2 is a cross-sectional view taken along line I-I of FIG. 1, which illustrates a cross-sectional structure of the second polarizing plate 14b. Referring to FIG. 2, the second polarizing plate 14b sequentially includes a first adhesive layer 20, a first passivation layer 21, a polarizer 22, a second passivation layer 23, a second adhesive layer 24, a λ/4 phase shift plate 25, a third adhesive layer 26, a Cholesteric Liquid Crystal (CLC) layer 27 and a third passivation layer 28. At this time, an upper surface of the first adhesive layer 20 is in contact to the lower substrate 10, and a lower surface of the third passivation layer 28 is in contact to the backlight unit 15. The first, second, and third passivation layers 21, 23 and 28 are formed of Tri-Acetyl-Cellulose (TAC).
In order to obtain the necessary thinness and lightness of an LCD module for a notebook PC in the LCD device having the aforementioned structure, a light-scattering means formed on a light-guiding plate of the backlight unit 15 is formed of three sheets. The light-scattering means receives the light emitted from the backlight, and uniformly scatters the received light to an entire surface of the LCD panel.
Generally, the light-scattering means is comprised of four sheets such as a lower light-diffusion plate, first and second prism sheets, and an upper light-diffusion plate. Recently, the light-scattering means using the three sheets, removing the upper light-diffusion plate to decrease the thickness of the LCD device. Thus, the light-scattering means is formed from a lower light-diffusion plate 15a and first and second prism sheets 15b. However, as compared to the light-scattering means using four sheets, the light-scattering means using only three sheets has problems with a backlight Mura phenomenon (referred to as Newton's Ring or Wet-Out). According to the backlight Mura phenomenon, rainbow-spots are generated on a screen when two glass substrates come in contact with each other, thereby generating spots on the screen during displaying of the picture image. As a result, the picture image is not smoothly displayed on the screen.
A key quality requirement for photomasks used in fabricating the display is the absence of Mura. Mura is caused by systematic deviations in the photomask and can be visible as stripes. Mura compromises the image quality of the finished display. Usually the deviations causing the Mura are very small, below a few hundred nanometers. While deviations of that size spread over a large area can be difficult to detect by measuring, the human eye can still see them due to its high sensitivity to systematic changes in gray scale. Laser repairs are often performed to correct such deviations, however, such repairs are difficult, time consuming, and costly as they require specialized equipment.
In order to solve the problem of the backlight Mura phenomenon in the related art LCD device as shown in FIG. 2, a diffusion process is performed on the polarizing plate by adding beads to the third adhesive layer 26 between the CLC layer 27 and the λ/4 phase shift plate 25. This diffusion process permits the backlight Mura phenomenon to be decreased. However, the luminance of the resulting LCD is smaller than that of an LCD having a polarizing plate in which the diffusion process is not performed. Also, if laser repair is to be performed, it is extremely difficult to focus on the layer of the LCD panel to be repaired when watching the lower substrate 10 under the microscope because of the large density of beads added to the third adhesive layer 26 for the diffusion process.
Specifically, one of polarizing plate characteristics, Haze, indicates the light-scattering intensity of transmitted light and reflected light. If the Haze value is small, brightness of the screen is greatly changed at portions such as the black matrix layer (i.e. the area excluding the light), so that it is hard to smoothly display the picture image. Meanwhile, if the Haze value is large, deterioration in the resolution ratio results. If the diffusion process is not performed on the polarizing plate, the Haze of the standard polarizing plate is about 0% and the luminance is about 30%. These values are such that the polarizing plate is susceptible to the backlight Mura phenomenon. However, if the conventional diffusion process is performed on the third adhesive layer, the Haze of the polarizing plate is about 80% and the luminance is only about 20%. While such a Haze prevents the backlight Mura phenomenon, this is an unacceptable decrease in luminance. In practicality, a Haze of at least 40% permits sufficient scattering to reduce the backlight Mura phenomenon to a negligible amount.